1. Field of the Invention
The present invention relates to a modified photocatalyst sol. More particularly, the present invention is concerned with a modified photocatalyst sol comprising a liquid medium having dispersed therein particles of a modified photocatalyst, wherein the modified photocatalyst particles are prepared by subjecting particles of a photocatalyst to a modification treatment with at least one modifier compound selected from the group consisting of different compounds each independently comprising at least one structural unit selected from the group consisting of a monooxydiorganosilane unit, a dioxyorganosilane unit and a difluoromethylene unit, and wherein the modified photocatalyst particles have a specific volume mean particle diameter. The present invention is also concerned with a modified photocatalyst composition comprising the above-mentioned modified photocatalyst sol and a functional substance. When a film containing a modified photocatalyst is formed on the surface of a substrate by using the above-mentioned modified photocatalyst sol or modified photocatalyst composition, not only can the modified photocatalyst be strongly immobilized on the surface of the substrate under moderate conditions without suffering a lowering of the activity of the modified photocatalyst, but also any of the film formed and the substrate covered with the film are not deteriorated by the action of the modified photocatalyst. Further, the above-mentioned film exhibits excellent properties with respect to transparency, durability, staining resistance, hardness and the like. Therefore, the above-mentioned modified photocatalyst sol and modified photocatalyst composition are extremely useful for, e.g., preventing dirt from adhering to the surfaces of various substrates, and preventing fogging from occurring on the surfaces of various substrates.
The present invention is also concerned with a film formed using the above-mentioned modified photocatalyst sol or modified photocatalyst composition, a functional composite comprising the film and a substrate covered with the film, and a shaped article produced by shaping the above-mentioned modified photocatalyst composition.
2. Prior Art
When specific types of substances are irradiated with light having energy which is larger than the energy gap (band gap) between the conduction band and the valence band of these substances, i.e., irradiated with light (excitation light) having a wavelength smaller than a wavelength corresponding to the band gap of these substances, the light energy causes an excitation (photoexcitation) of the electrons in the valence band, so that an electron and a hole are, respectively, generated in the conduction band and the valence band. By using the reducing activity of the electron generated in the conduction band and/or using the oxidizing activity of the hole generated in the valence band, various chemical reactions can be performed.
That is, when the above-mentioned substances are irradiated with excitation light, they can function as catalysts. Therefore, the above-mentioned substances are called “photocatalysts.” As a most representative example of such photocatalysts, titanium oxide is known.
As examples of chemical reactions catalyzed by the photocatalysts, there can be mentioned oxidative decomposition reactions of various organic substances. Therefore, when the photocatalysts are immobilized on the surfaces of various substrates, it is possible to cause various organic substances which have adhered to the surfaces of the substrates to undergo oxidative decomposition by light irradiation.
On the other hand, it is known that, when photocatalysts are irradiated with light, the hydrophilicity of the surface of the photocatalysts is increased. Therefore, when photocatalysts are immobilized on the surface of various substrates, it is possible to increase the hydrophilicity of the surface of the substrates by light irradiation.
In recent years, studies have been made for applying the above-mentioned properties of photocatalysts to various fields, such as the field of environmental clean-up, fields in which it is necessary to prevent dirt from adhering to the surface of various substrates, and fields in which it is necessary to prevent fogging from occurring on the surfaces of various substrates. For enabling such applications, the method for immobilizing photocatalysts on the surface of various substrates has a great importance.
With respect to the method for immobilizing a photocatalyst on the surface of a substrate, various proposals have been made. For example, Unexamined Japanese Patent Application Laid-Open Specification No. 60-044053 discloses a method for immobilizing a photocatalyst on the surface of a substrate, in which a thin film of a photocatalyst is formed on the surface of a substrate by sputtering.
Of these methods which have been proposed, one which has been attracting attention as being especially advantageous is a method for immobilizing a photocatalyst on the surface of a substrate, in which a composition containing a photocatalyst is coated on the surface of a substrate to thereby form a photocatalyst-containing film thereon.
In the above-mentioned method for immobilizing a photocatalyst by coating, it is required:                {circle around (1)} that the photocatalyst be strongly immobilized on the surface of the substrate without suffering a lowering of the activity of the photocatalyst, and        {circle around (2)} that any of the film formed and the substrate covered with the film formed be not deteriorated by the action of the photocatalyst.        
Further, for broadening the application field of the above-mentioned method, it is desired:                {circle around (3)} that the immobilization can be performed under moderate conditions (for example, at room temperature to about 100° C.), and        {circle around (4)} that the film formed exhibits excellent properties with respect to transparency, durability, staining resistance, hardness and the like.        
For immobilizing a photocatalyst by coating, various methods have conventionally been proposed.
For example, Unexamined Japanese patent Application Laid-Open Specification No. 60-118236 discloses a method in which a sol containing a photocatalyst precursor (for example an organotitanate) is coated on the surface of a substrate, and the resultant film is calcined to thereby convert the photocatalyst precursor into a photocatalyst while immobilizing the formed photocatalyst on the surface of the substrate. However, this method includes a step for forming crystalline microparticles of a photocatalyst, wherein this step requires calcination at high temperatures. Therefore, this method has a disadvantage in that, when the substrate has a large surface area, it is difficult to perform the immobilization of the photocatalyst.
As a method which uses a sol containing a photocatalyst (and which, hence, does not require a step for forming crystalline microparticles of a photocatalyst), Unexamined Japanese Patent Application Laid-Open Specification No. 6-278241 discloses a method for immobilizing a photocatalyst on the surface of a substrate, in which a water-peptized sol of titanium dioxide is coated on the surface of a substrate. However, since a titanium oxide sol cannot exhibit a film-forming property under moderate conditions, this method also requires calcination at high temperatures. Further, the film obtained by this method is brittle and easily broken and comes off from the substrate, so that it is impossible to allow the photocatalyst to exhibit its effect at the surface of the substrate.
Also, methods have been proposed in which a resin coating material containing a photocatalyst is coated on the surface of a substrate. For example, each of Unexamined Japanese Patent Application Laid-Open Specification Nos. 7-171408 and 9-100437 discloses a method in which a photocalyst is incorporated into a resin coating material containing, as a film-forming element, a resin which is unlikely to be decomposed by the action of a photocatalyst, such as a fluoro resin or a silicone resin, and the resultant photocalyst-containing coating material is coated on the surface of a substrate. However, in the resin coating materials used in these methods, the dispersion of the photocatalyst is poor and, therefore, the coating materials become white turbid. Further, for obtaining a good film by these methods, it is necessary for the coating materials to contain a large amount of a resin, and this use of a large amount of a resin poses a problem in that the photocatalyst is embedded in the film and, hence, cannot exhibit a satisfactory activity.
As a method for solving these problems, Unexamined Japanese Patent Application Laid-Open Specification No. 9-314052 discloses a method which uses a combination of a resin coating material and photocatalyst particles having a controlled wettability with respect to the solvent contained in the resin coating material. In this method, the resin coating material is first coated on the surface of a substrate and, then, before the curing of the coating material is completed, the above-mentioned photocatalyst particles are coated on the coating material. However, this method is disadvantageous in that not only is the operation cumbersome, but also a homogeneous and transparent film cannot be obtained. The above-mentioned patent document also discloses a method intended to simplify the operation, specifically a method in which a mixture of a resin coating material and photocatalyst particles having a controlled wettability with respect to the solvent contained in the resin coating material is coated on the surface of a substrate. However, the photocatalyst cannot be prevented from being embedded in the film by using photocatalyst particles having a controlled wettability with respect to the solvent, and most of the photocatalyst particles are completely embedded in the film and, hence, cannot exhibit a satisfactory activity.
Thus, no method has yet been known which is for immobilizing a photocatalyst on the surface of a substrate by coating and which satisfies all of the above-mentioned requirements {circle around (1)} to {circle around (4)}.